Agricultural machines such as high-clearance sprayers are getting larger and more complex, with some sprayers having four-wheel steering to improve maneuverability, such as when making tight turns in a field's headlands. Such four-wheel steering systems typically have a controller that electronically controls a single steering valve which meters hydraulic fluid to both hydraulic rear-steering cylinders to correspond to a front-steering input. However, collision-induced rear wheel misalignment and/or steering errors at one rear wheel can be difficult for the controller to manage. Furthermore, although four-wheel steering is beneficial, there are times when it may be desirable to safely prevent four-wheel steering, such as when traveling on a road at high speed. A need therefore exists for an improved system which eliminates one or more of the foregoing disadvantages.